1. Field of the Invention
The present invention generally relates to screen printing devices and screen printing methods.
More specifically, the present invention relates to a screen printing device for printing paste and a squeegee screen printing method.
2. Description of the Related Art
FIG. 1 is a schematic view for explaining related art screen printing using a solder paste. FIG. 1(a) shows the screen printing in progress and FIG. 1(b) shows the completed screen printing.
A squeegee 50 is configured to move while pressing a solder paste 51 downward and pushing the solder paste 51 forward. The solder paste 51 is a printing subject made of particles of solder and particles of flux. Openings (or holes) 53 are provided at a portion of a screen printing plate (metal mask) 52 whose base is metal, which is the portion where the solder paste 51 is to be printed. Solder pastes 54 are provided in the openings 53. A printed wire 55 is provided on a surface of a printed board 56. The squeegee 50 is driven forward and backward by a driving mechanism (not shown in FIG. 1).
In the above-mentioned screen printing device using the solder paste, first, the screen printing plate (metal mask) 52 having thickness of 0.15 mm is positioned on the printed board 56 which is a printing subject. Holes are provided at parts where the paste is to be printed. A piece of the squeegee 50 is inclined at a certain angle in a moving direction. The squeegee 50 is advanced while the squeegee 50 presses the solder paste 51 in a screen printing plate 52 direction (downward). By pressing the solder paste 51 onto the printed board 56 via the openings 53 of the screen printing plate 52, the solder paste 51 is forced into the openings 53. After that, by removing the screen printing plate 52 from the printed board 56 slowly, as shown in FIG. 1(B), the solder paste 51 provided in the openings 53 remains printed on the printed board 56.
In order to exert a force for pressing the solder paste 51 downward, the squeegee 50 is normally provided so as to be inclined at approximately 20 through 45 degrees. As a result of this, an upper part of the squeegee 50 projects in the moving direction compared with a position where the squeegee 50 comes in contact with the screen printing plate 52. In addition, when the squeegee 50 is advanced, the squeegee 50 is pressed downward by an outside driving force so as to stand against a reaction force from the solder paste 51 having high viscosity and density. Since the squeegee 50 should endure against a printing pressure (a force for driving downward) when the squeegee 50 is inclined forward, a certain degree of strength is required. Since the printing pressure in the downward direction is normally applied to the printed board 56 via the screen printing plate 52, a backup block or backup pin (not shown in FIG. 1) is provided under the printed board 56 so that the printed board 56 is supported.
The printing pressure is an important control item in a solder paste screen printing device. Accordingly, in recent printing devices, even if the printed board is curved, “feedback control” is applied, where pressure is detected and the printing pressure is controlled so as to be increased or reduced based on the detected output, so that a certain printing pressure can be maintained. In order to increase the ability to adhere to the screen printing plate 52 and follow a slight curve or deformation of the printed board 56, the squeegee 50 is made of urethane rubber or a flexible metal plate having thickness of approximately 0.2 mm through approximately 0.5 mm (normally stainless).
In the example shown in FIG. 1, printing of the solder paste 51 is performed in a single direction (for example, from right to left). As a technique which is an improvement of the example shown in FIG. 1, an example shown in FIG. 2 is suggested. The example shown in FIG. 2 is discussed in Japanese Laid-Open Patent Application Publication No. 2006-167992. In this example, two squeegees 50 are provided in a right and left symmetric manner to print in opposite directions. Bi-directional printing can be done such as from right to left or from left to right.
FIG. 2 is a view for explaining a case where two squeegees 50a and sob are used. A numerical reference 35 indicates a mask frame. As shown in FIG. 2(a), the second squeegee Sob moves from a home position downward in a vertical direction so as to come in contact with a screen printing plate 52. Then, as shown in FIG. 2(b), each of the squeegees 50a and 50b starts moving to a left side in a horizontal direction. By this movement, a solder paste on the screen printing plate 52 fills piercing openings (not shown in FIG. 2) formed in the screen printing plate 52. When the squeegees 50a and 50b arrive in the vicinity of an opposite end part of the screen printing plate 52, namely a position shown in FIG. 2(c), the squeegee 50a moves downward in the vertical direction and the squeegee 50b moves upward in the vertical direction so that the disposition shown in FIG. 2(d) is formed. The squeegee 50b moves to the height of the home position so that the squeegee 50a comes in contact with the screen printing plate 52. An electronic circuit board is adhered on a lower surface of the screen printing plate 52. After one pass of the squeegees 50a and 50b is completed, the electronic circuit board is removed from the screen printing plate 52. By this removal, the solder paste is transferred onto the electronic circuit board. After this, a new electronic circuit board is attached and the squeegees 50a and sob are moved to a right side in the horizontal direction as shown in FIG. 2(d). When the squeegees 50a and sob arrive in the vicinity of the opposite side end part of the screen printing plate 52 as shown in FIG. 2(e), the solder paste provided on the screen printing plate 52 is squeezed by the movement of the squeegee 50a again so that a pattern of the solder paste is printed on a new electronic circuit board.
In the related art cases, since the printing pressure of the squeegee is finally applied to the printed board, it is necessary to support the board from the lower surface of the board. On the other hand, in recent years, two-surface mounting boards where components are mounted on both upper and lower surfaces of the board have been mainly used. When a second component is mounted on one surface of the board after a first component is mounted on the other surface of the board, the first component is provided on the lower surface of the board and therefore the lower surface other than an area where the first component is provided need to be supported by a backup pin or the like.
FIG. 3 is a view for explaining screen printing applied to a surface of a board having a rear surface where components are mounted. In FIG. 3, parts that are given the numerical references 50 through 52 and 56 correspond to the parts having the same numerical references of FIG. 1. In other words, the numerical reference 50 represents the squeegee 50, the numerical reference 51 represents the solder paste, the numerical reference 52 represents the screen printing plate (metal mask), and the numerical reference 56 represents the printed wiring board. A base 57 is configured to support the printed board 56. Backup pins 58a through 58g are provided on the base 57. Components 59, such as an integrated circuit or resister, are provided on a rear surface of the printed board 56. As shown in FIG. 3(a), one end of each of the backup pins 58 is situated in a part of the rear surface of the printed board 56 where the components 59 are not mounted. Other ends of the backup pins 58 are fixed on the base 57. Accordingly, it is possible to prevent the printed board 56 from being bent downward when a printing pressure is applied in a lower direction for printing the solder paste by the squeegee 50.
However, in a part where the components are provided in a line, namely a section between the backup pins 58e and 58f shown in FIG. 3(a), the printed board 56 is curved downward due to the downward printing pressure of the squeegee 50. In this case, as shown in FIG. 3(b), the squeegee 50 cannot follow the bending and therefore the printed board 56 may be damaged if pressure is applied to the squeegee 50 in a depresses position, and thereby thin solder paste 51 remains on the screen printing plate 52.
It is normal practice that the solder paste 51 may not remain on the screen printing plate 52 other than inside of the openings after the squeegee 50 passes. However, if the solder paste 51 remains, the solder paste 51 may cover the openings of the screen printing plate 52 and therefore the solder paste inside the openings may not be printed (transferred). In other words, when the screen plating plate 52 is removed from the printed board 56 after the printing, the solder paste provided in the openings 52 may be removed together with the remaining solder paste.
In order to prevent this problem, the printing pressure is decreased so that the curvature of the board may be reduced. However, in the case where the printing pressure is decreased, due to the reaction force of the solder paste when the squeegee is advanced, it is not possible to remove the solder paste sufficiently so that the solder paste may remain on the screen printing plate 52.
In the method shown in FIG. 2, two squeegees are used differently, namely for printing operations in a right direction and a left direction. By a printing operation in a single direction, a single electronic circuit board can be printed. Accordingly, the method shown in FIG. 2 does not work as means for deformation of the printed board.
As shown in FIG. 3, in the case where the components are mounted on one surface of the two-surface mounting printing board with high density and then the printing paste is printed on the other surface of the board so that the printed board cannot be sufficiently supported (backup) from a lower side, removal of the solder paste remaining due to the curvature of the printed board and the curvature of the board have a trade-off relationship. This relationship is related to viscosity of the solder paste, an angle of the squeegee, hardness, advancing velocity of the squeegee or the like and therefore it is difficult to control. The same problem may occur in a case where not only the solder paste is printed on the printed board but also a printing paste (printing member of the wiring pattern) is printed on the printed board.